1. Field of the Invention
The present invention relates to a method for enabling multi-channel signaling in a communication network, in particular in a vehicular ad hoc network, wherein said communication network comprises a multitude of communication nodes, wherein the communication among said communication nodes is performed by way of sending and receiving messages on communication channels, wherein said communication channels include a control channel and at least one service channel.
2. Description of the Related Art
Vehicular communication is considered as key technology for Intelligent Transport Systems (ITS) because the vehicular communication has the capability to increase road safety and traffic efficiency. For this purpose, the mature, inexpensive, and widely available 802.11 wireless LAN technology appears very attractive. In the field of vehicular communication, vehicles are equipped with wireless transceivers and can spontaneously form an ad hoc network among them. Vehicles acting as network nodes can use the ad hoc network to communicate with each other in order to support safety applications such as cooperative collision warning.
Recognizing the potential of vehicular communication, the European Commission has recently allocated a 30-MHz frequency band (5875-5905 MHz) for safety-related communication of Intelligent Transport Systems (Commission Decision of Aug. 5, 2008 on the harmonized use of radio spectrum in the 5875-5905 MHz frequency band for safety-related applications of ITS, 2008/671/EC). Additional frequency bands for vehicular communication have been planned for the future. While the European Commission has not specified how this frequency band will be used, based on the current status of standardization activities in ETSI TC ITS (European Technical Standards Institute Technical Committee Intelligent Transport Systems), it is expected that this frequency band will be divided into one 10-MHz control channel (CCH) and two 10-MHz service channels (SCH1 and SCH2). Exemplary it is referred to the Paper Long Le et al., “Analysis of Approaches for Channel Allocation in Car-to-Car Communication”, 1st International Workshop on Interoperable Vehicles (IOV 2008), pages 33-38, Zurich, Switzerland, March 2008, describing the analysis for such channel allocation. With additional frequency bands to be allocated in the future, it is expected that there will be more service channels. Therefore, vehicular communication in Europe will feature multi-channel operation.
Further, as the automotive industry requires that a vehicle is constantly able to receive messages sent on the control channel, the most appropriate solution according to currently and short/mid-term available hardware consists of a dual transceiver communication system including two transceivers operating in this frequency band. One of these transceivers will operate in the control channel that is dedicated to the exchange of periodic messages and event-driven warning messages for active safety applications. The other transceiver will operate alternately on service channels and will be used for other ITS-related communication purposes.
In the USA a 75-MHz frequency band (5.855-5.925 GHz) has been allocated for Dedicated Short Range Communications (DSRC), which is divided into a single CCH and six SCHs. The multi-channel operation follows the IEEE P1609.4 standard which prescribes that a single transceiver constantly switches between the CCH and one of the SCHs. There is no “always on” channel, but all communication nodes will periodically switch to the CCH.
Consequently, there are multiple wireless channels for inter-vehicular communication both in the USA and Europe and furthermore, there are two types of channels for vehicular communication systems. The first channel type is the control channel, and there will be an envisaged period of time that all communication nodes will switch their transceivers onto the control channel. In case there are a multitude of transceivers, one transceiver will be always tuned to the control channel. In case that a single transceiver is provided, the transceiver uses channel switching between the control channel and other channels. The second type of channel is the service channel, which is used by a communication node only occasionally and wherein it is not expected that all communication nodes will tune their transceivers on the service channel at expected points of time.
The IEEE 802.11 standard (IEEE Std 802.11™-2007, IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications) prescribes distributed signaling at the MAC layer, which is used to coordinate beacon generation in an IBSS (Independent Basic Service Set). All WLAN stations in an IBSS synchronize their beacon generation by adopting the same beacon period when joining the IBSS. Such signaling can be employed for a signal transceiver system running on a single channel.
A multi-channel signaling mechanism is described in WO 2007/021158 A1. The disclosed technique addresses a signaling method and system for indicating and selecting a channel suitable for a data transmission, whereby the suitable channel is indicated by means of a channel index embedded in the Request-To-Send (RTS) and Clear-To-Send (CTS) frames defined in an IEEE 802.11 WLAN. Although the proposed technique addresses multiple channels, it is only applicable to single transceiver systems. Further, this multi-channel signaling mechanism operates on a per-packet basis and incurs a considerable signaling overhead.
The IEEE 1609.3 standard (IEEE Std 1609.3™-2007, IEEE Trial-Use Standard for Wireless Access in Vehicular Environments (WAVE)—Networking Services) prescribes that applications can choose to send their traffic in the context of a WBSS (WAVE basic service set). Such a signaling method is based on IEEE 1609.4 systems that are described in IEEE Std 1609.4™-2006, IEEE Trial-Use Standard for Wireless Access in Vehicular Environments (WAVE)—Multi-channel Operation. The mentioned method features a single transceiver and multiple channels. The system requires transceivers from all communications peers synchronize to the UTC (Universal Time Coordinated) time and defines a “Sync Interval” within which each transceiver will switch between the single CCH and one of the multiple SCHs. The same procedure will be repeated in each “Sync Interval”. A WBSS is established to support traffic to/from specific applications, and its presence is announced for other devices with compatible applications to join. Devices take the role of either provider or user on a given WBSS. The provider generates messages called WAVE announcements to inform other devices of the existence of the WBSS and the presence of the associated application service(s). The user's role is assumed by any devices that join the WBSS based on receipt of the announcement. The WBSS is initiated at the request of the application at one device (the provider), and announced on the CCH. Since WAVE announcements are only received by single hop communication peers, the resulting communication using the WBSS is also limited to single hop communication. In addition, if the number of nodes is high, the number of WAVE announcements may also be high. Although IEEE 1609.3 is based on IEEE 1609.4 with single transceiver, its method may also be applied to multi-transceiver systems. However, the main limitation of IEEE 1609.3 is that it only allows single-hop communication.
Applications that are used for road safety or traffic efficiency normally are not interested in which communication channel will be used. The common way is that applications send requested service messages to the network layer, and the network layer will send application messages on a certain channel based on certain mapping rules or network management functions. The mapping of an application to a communication channel can be based on a policy, in that a certain application shall only run on a certain channel. Or the applications are mapped to a channel dynamically based on network management functions due to load balancing reasons or interference in channels. For certain applications, such as critical safety applications, the messages are simply sent on the control channel. However, if an application should use a service channel, the network layer cannot determine if other communication peers are also using the same service channel since the service channel is not always on.
In addition to communication channel, vehicular communication may involve different access technologies, different transport and/or network protocols, and different quality of service requirements on the communication, which can be described as communication profile.
Therefore, there is a need to signal the intended usage of a service channel, more generally, the communication profile, in order to get other communication peers involved. The signaling should be done in a way to hide routing and channel related details from applications. Since many applications require vehicular communication to cover a certain area, which goes beyond the coverage of single hop communication, the signaling shall also have the capability to reach beyond a single hop. Moreover, since the communication scenario considered in this invention is at the network layer and may extend to several hops, the signaling for multi-channel using MAC layer protocols will not be appropriate since it only covers a single hop.
There are a number of challenges for multi-channel signaling:                1. Signaling is in a distributed ad hoc manner without any central coordination.        2. There are potentially a large number of nodes, and each may start multi-channel signaling. But there is very limited bandwidth available in the control channel, thus it must be very efficient in terms of bandwidth usage.        3. The signaling needs to go beyond a single hop without complex forwarding mechanisms.        
For the multi-transceiver, multi-channel vehicular communication system in Europe, there is no method for signaling multi-channel usage. ETSI TC ITS still has not addressed this problem so far. Several possibilities are noted in the following that reflecting typically what approaches in this area are followed.
One possible approach is to follow IEEE 1609.3: Using one transceiver on the CCH to send messages similar to “WAVE announcements” to signal the intended usage of a certain service channel. As already mentioned, this approach does not support multi-hop communication.
Another problem with the IEEE 1609.3 approach is the bandwidth limitation. In Europe, the CCH will mainly be used for important active safety applications, and it has to carry periodic and event-driven messages. To allow each communication node send messages similar to “WAVE announcements” whenever there is a need to use a SCH would increase the number of packets and thus the load on the CCH. Hence, the IEEE 1609.3 approach is not an efficient solution.
To support multi-hop communication, a straightforward way is to forward a signaling message for multiple hops in order to extend the dissemination area. There are already methods available for forwarding, such as topologically broadcast, which forwards a message for a predefined number of hops, and geographical broadcast, which forwards a message to a geographical area. However, such multi-hop forwarding often leads to many packet retransmissions, and is not suitable on the CCH.